MARSHFIELD – The Massachusetts Audubon Society is adding more than 100 acres of environmentally important coastal marshland to the Daniel Webster Wildlife Sanctuary.

The land transfer, which involves property previously held by Marshfield Airport, has been in the making for more than a decade. It is part of the $15.34 million runway-safety improvement project at George Harlow Field, and brings the total amount of protected land at Daniel Webster to nearly 600 acres.

The sanctuary includes uplands, forest, ponds, broad fields and now marshlands nurtured by the tidal Green Harbor River.

Sue MacCallum said the transfer has been in the works since before she became Audubon’s South Shore sanctuaries director eight years ago.

“We’ve been working with the town, airport officials, the DEP, other state and federal agencies,” she said Monday. “... It’s fabulous to see it come together.”

The newly reopened runway at Marshfield Airport was shifted 190 feet west of the previous surface, widened by 25 feet and extended by 300 feet. An additional 300 feet of paved safety buffer was added at each end, providing 3,600 feet for takeoffs and 3,900 feet for emergency landings.

The Federal Aviation Administration provided an $11.34 million grant for improvements to the town-owned airport, which is managed by Shoreline Aviation. The state pitched in $1.4 million, and voters at a special town meeting in 2011 approved $200,000.

Acquired as part of the project, the additional habitat supports an array of animal and plants species, including the vulnerable Eastern box turtle.

“(The airport) needed access to some of our land for the vegetative clearing so that planes have the clearance needed at the end of the runway,” MacCallum said. “They did some cutting on our land, but a very small amount.”

MacCallum said marshland is an important habitat to preserve, especially since it plays multiple mitigation roles. Most importantly, it can act as a sponge to absorb storm surges across Green Harbor, protecting residences, businesses and people.

“A lot of people don’t see marshland as being very valuable because it’s not easy to develop,” she said. “But in 50 years, you don’t know what will happen, so it’s great that it will be protected permanently.”